Dysregulation of RNA pathways is emerging as a central feature of degeneration of motor neurons. Amyotrophic lateral sclerosis (ALS) is the most common human motor neuron degenerative disease with a fatal outcome and without a cure. Approximately 20% of ALS patients also develop Frontotemporal Lobar Degeneration (FTLD), characterized by dementia due to neuronal degeneration and often with the presence of ubiquitinated protein aggregates (FTDL-U). Recent studies have shown that mutations of two RNA Binding Proteins (RBPs) known as TDP-43 (or TARDBP) and FUS (or FUS/TLS) cause familial ALS (FALS) and FTLD-U;and protein aggregates containing TDP-43 or TDP-43 carboxy-terminal (C-terminal) fragments are detected in the majority of sporadic ALS cases and in many cases of FTLD-U (FTLD-TDP), further underscoring the paramount importance of RBPs and of RNA dysregulation in neuronal degeneration. The RNA targets and function of TDP43 and FUS in motor neurons are unknown. Furthermore, it is unknown how mutations of TDP-43 and FUS lead to neuronal degeneration. In this application we propose to develop novel experimental systems and strategies to investigate the function of TDP-43 and FUS in RNA pathways and to dissect pathogenetic mechanisms involved in motor neuron degeneration by TDP-43 and FUS patient mutants. In Aim 1, we will establish and characterize embryonic stem cell derived motor neuron culture systems to investigate TDP-43 and FUS. We will characterize the cellular and molecular effects of mutant TDP-43 and FUS in cultured motor neurons and in the motor neurons of ALS patients. In Aim 2, we will identify RNA target for TDP-43 and FUS in human brain and motor neurons and we will investigate whether TDP-43 and FUS patient mutations result in RNA dysregulation in motor neurons. We expect that our studies will lead to the development of novel experimental systems and will provide critical insights in the function of TDP-43 and FUS in motor neurons and the role of patient mutations in neuronal degeneration. We also anticipate that the outcome of this exploratory R21 grant application may provide important insights that will guide further investigations in therapeutic strategies to combat this lethal disease. PUBLIC HEALTH RELEVANCE: In this application we propose to develop novel experimental systems and strategies to investigate how motor neurons degenerate in ALS, a devastating neurodegenerative disease. We anticipate that our studies may provide important insights that will guide further investigations in therapeutic strategies to combat this lethal disease.